The basic signal used in SONET is referred to as a STS-1 (Synchronous Transport Signal level 1), which has a bandwidth of 51.84 Mb/s. As is well-known, a STS-1 includes overhead information (e.g., a payload pointer) and payload information. It can be appreciated that in certain instances there is need to transport payload information (signals) at a rate/bandwidth greater than a STS-1. That is, a larger envelope is needed to transport the signals. This is particularly true for video signals, e.g., High Definition TeleVision (HDTV), Asynchronous Transport Mode service and similar services (i.e., video conferencing). To meet that need, N STS-1's are grouped together to form a bandwidth (envelope) larger than that provided by a single STS-1, in which N STS-1s are identified as a STS-N circuit. Also, for video services and like service requirements, there is a need to concatenate STS-1s to preserve the integrity of the transmission--which means that the larger envelope is transported as a single entity. Concatenation is identified in a SONET transport system by the letter c, e.g., STS-Nc, where N&gt;1. Thus, N contiguous STS-1 channels need to be assigned to a STS-Nc signal.
Since a STS-Nc is transported as a single entity, only one payload pointer is needed. The unused pointers may then be used to identify the concatenated signals. The first payload pointer in the first of the N STS-1s contains a normal pointer value, and each of the succeeding payload pointers instead contains a concatenation indicator.
Typically, a so-called Operations System (OS) is the facility that assigns/provisions N contiguous STS-1s in a SONET transport system to handle a particular concatenated signal. If such provisioning is not done correctly, then there is good chance that a concatenated signal may not be transported as a single entity over a group of contiguous STS-1 s, which may cause the system to generate a series of alarms that do not specifically identify the problem. As such, the OS may not be able to quickly resolve the problem. In certain instances alarms may not be generated even though such provisioning is incorrect. For example, consider the case where two groups of contiguous STS-1 s have been provisioned to transport respective concatenated signals and that a STS-1 of one group is inadvertently swapped with a STS-1 of the other group such that the pointer protocol for the concatenated signals is still preserved. In that case, the structure of both concatenated signals would appear to be correct, and, thus, the system would not generate an alarm. In fact, only the respective recipients of the signals would be able determine that the integrity of the received signal had been compromised.